28. July 2008

Ground temperatures at karst lakes on Vardeborgsletta, western Spitsbergen

Contact person for additional information: Hanne H. Christiansen

Text and photos by Ole Humlum

Karst lakes on Vardeborgsletta July 27, 2004 , looking southeast. The ground temperatures are measured in vertical profiles near the outlet (left arrow) and in the plateau above the valley (right arrow). The plateau site is located about 100 m to the right of the photo. The position of the subterraneous water outlet is indicated by the red dot (see photo below). A series of talus-derived rockglaciers lining the foot of the Vardeborg mountain is seen in the background.

Vardeborgsletta is located east of Kapp Linné at the west coast of Spitsbergen , on the southern side of the entrance to the main fjord system, Isfjorden. The Vardeborgsletta can be considered part of the strandflat, covered by Holocene beach deposits. The topography of Vardeborgsletta is characterised by a number of north-south trending linear valleys with lakes. These valleys are unusual as they have a irregular length profile with lakes, wherefore they can not be eroded by normal rivers. In addition, some of the valleys do not drain towards the ocean, but inland. There are also examples of subterraneous drainage (se photo below). 

Subterraneous drainage of the valley seen in the photo above, July 27, 2004. Water draining from the lakes disappears into the ground at the inland end of the valley.

The local bedrock below this part of Vardeborgsletta is Permian limestone (290-270 mill. years old), a bedrock type which shows increasing solubility with decreasing temperature, in contrast to most other bedrock types. It has therefore been suggested that the valleys and lakes in Vardeborgsletta are karst phenomena, produced by chemical solution (Åkerman 1980; Salvigsen and Elgersma 1985). The linear valleys itself may indicate the position of active, geological fault lines parallel to the west coast of Spitsbergen.

Permafrost is expected to characterise the area near Kapp Linné, a situation which would tend to hinder the development of karst phenomena, as most water would be frozen and therefore not available for chemical processes. Ongoing geological faulting activity, however, may produce enough frictional heat to prohibit the development of permafrost along the fault lines, keeping ground temperatures near or above freezing, thereby enabling the presence of unfrozen ground water for prolonged periods each year. If so, this would make rapid chemical solution of the limestone possible along the fault lines, with associated development of karst features like sinkholes and subterraneous drainage.

To investigate the viability of this hypothesis, at 25 July 2005 two ground temperature profiles were therefore established on Vardeborgsletta, near the bottom of one of the valleys and on the undisturbed surface in between valleys, respectively (see photo at top of this page). Four thermistors were installed in each profile from the surface to about 150 cm depth. Ground temperatures have since been recorded hourly. The results are shown in the two diagrams below

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Ground temperatures measured in the undisturbed surface near one of the valleys in Vardeborgsletta (see photo at top of this page). The temperature at 145 cm depth is 2-3^oC above freezing for only a short period each summer, and presumably indicates the existence of permafrost at slightly greater depth.

Ground temperatures measured near the valley bottom near the place of subterraneous drainage from a row of karst lakes (see photo at top of this page). The temperature at 150 cm depth is 6-8^oC above freezing for a long period each summer, and presumably indicates the absence of permafrost at the bottom of the valley.

Our ground temperature measurements clearly show that the ground is warm near axis of the investigated valley, compared ground below the undisturbed plateau surface nearby. This potentially makes chemical solution of limestone below the valley more important than below the plateau surface between valleys.

We have thus not been able to falsify the hypothesis on local absence of permafrost controlling karst development at Vardeborgsletta, and the idea therefore remains a viable hypothesis. Vardeborgsletta may thus represent a fine example of how deep-seated tectonic processes (faulting) may influence both the distribution and thickness of permafrost and modern landscape development (geomorphology).

References:

Salvigsen, O. and Elgersma, A. 1985. Large-scale karst features and open taliks at Vardeborgsletta, outer Isfjorden, Svalbard . Polar Research, 3, 145-153.

Åkerman, H.J. 1980.  Studies on periglacial geomorphology in West Spitsbergen . Meddl. Lund Universitet Geogr. Inst. Avhandl., 297 pp.